The present invention relates generally to test specimens and methods for evaluating the behavior of a liquid flowing in a capillary channel driven by capillary forces. In particular, the present invention relates to a capillary test specimen, method, and system for in-situ visualizing and quantifying the flow of liquid polymers or molten solders or braze alloys or similar filer materials in capillary channels and gaps, including the formation of fillets at two free surfaces that have been configured to represent realistic joint geometries.
Many industrial processes rely on having predictable and reliable flow of liquids inside capillary channels and gaps, including proper formation of fillets at free surfaces. These processes include low temperature operations (e.g., polymer underfilling of surface-mounted microelectronic packages, polymer encapsulation, polymer injection casting or transfer molding, and secondary oil recovery), as well as high temperature operations (e.g., soldering, brazing, and casting of liquid metals). Brazing has been defined as a group of joining processes that produces coalescence of materials by heating them to a suitable temperature and by using a filler metal having a liquidus temperature above 450 C and below the solidus temperature of the base materials to be joined, whereby the filler metal is distributed between the closely fitted surfaces of the joint by capillary attraction. Soldering, a similar process to brazing, is defined to occur below 450 C. Brazing differs from welding in that in braze processing the intention is to melt only the braze filler metal and not the base materials. Good wetting of the base materials by the liquid filler metal is required to provide intimate contact between them, and to develop the necessary chemical bond for good joint strength.
A variety of test specimens and method have been used to evaluate the wetting properties of a liquid on a solid surface (e.g., sessile drop test, immersion wetting balance, closed capillary flow in a tube, open capillary flow in an open channel or groove, and capillary flow between two closely-spaced parallel plates).
In a sessile drop test, the liquid droplet assumes an equilibrium shape that is dictated by surface free energy considerations at the liquid-vapor interface, the solid-vapor interface, and the solid-liquid interface. The boundary between wetting and non-wetting conditions is generally taken at a contact angle, xcex8contact, equal to 90 degrees. For xcex8contact less than 90 degrees, wetting occurs; while xcex8contact greater than 90 degrees a represents a non-wetting condition (i.e., the lower the angle, the better the wetting). The sessile drop test, however, provides no information about the dynamic (i.e., time-dependent) flow of liquids through a capillary channel, or about the formation of fillets at free surfaces.
In industrial settings, the wetting performance of molten solder is commonly evaluated using commercially available wetting balances. A typical wetting balance suspends a specimen from a weighing device (such as a micro-balance or load cell), then a crucible containing the molten material is lifted and immerses the bottom portion of the specimen into a molten solder or braze pool to a known depth. By accurately measuring the force applied to the specimen during immersion in the pool, the wetting force and times can be determined. Using analytical expressions, the wetting angle can be calculated. An immersion balance, however, does not provide access for visualizing the flow of solder through a capillary channel, or for visualizing the formation of fillets at free surfaces.
In another test specimen, capillary flow of a liquid between two closely spaced parallel plates can be studied by measuring the change in capacitance between the two plates as the liquid flows from one side to the other. This test is commonly used to evaluate the flow behavior of liquid polymers that are used for underfilling surface-mounted microelectronic devices. This test specimen, however, is not suitable for use with electrically conductive liquids, such as solders and brazes. Also, the geometry of two parallel plates does not simulate the geometry of complex braze joints and the associated fillets that form thereon.
The flow of liquids inside a closed capillary channel, such as a simple tube, provides a simple relationship between the liquid surface tension, the capillary radius, the contact angle, and the bulk liquid viscosity. However, opaque tubes block visual access for observing the moving liquid front. Also, a simple tube does not have the complex geometry representative of realistic joint geometries.
An open capillary test specimen and method has been disclosed by Rye, et al, in U.S. Pat. No. 5,792,941, xe2x80x9cMeasurement of Surface Tension and Viscosity by Open Capillary Techniquesxe2x80x9d. Here, an open capillary channel is provided, such as a V-shaped groove, on a flat, wettable surface. The test specimen has timing marks adjacent to the V-groove, and a source marker in which liquid to be tested is deposited. The capillary flow of liquid as it passes by the timing marks is recorded by a video camera looking down on the specimen. Image processing software is subsequently used to determine the flow time and velocity of the moving liquid front. This measurement of the flow time and velocity can be analytically related to the ratio of surface tension-to-viscosity for the liquid (knowing the groove depth, the groove angle, and the liquid/solid contact angle). This test specimen, however, does not provide the complex geometry representative of realistic joint geometries.
With the exception of two closely-spaced parallel plates, none of the test specimens described above are suitable for studying two different materials in contact with the molten solder or braze (e.g., as found in a metal-to-ceramic joint, or polymer underfill between printed wiring board material and a silicon die).
The need remains, therefore, for a capillary test specimen that allows easy visual access from an open side to observe a cross-section of a braze or solder joint, which has a geometry that closely simulates a realistic joint geometry. Such a test specimen should allow easy access for a high-speed video or digital camera to observe the liquid in-situ as it melts (e.g., for a solder or braze), flows from a reservoir (i.e., source), through an open capillary channel between two surfaces having a controlled capillary gap, and into an open region where it subsequently forms a fillet on free surfaces, where the geometry of the free surfaces have been configured to accurately simulate realistic joint geometries. Also, such a test specimen should be capable of simulating liquid flow between surfaces made of two different materials, e.g., a metal surface and a ceramic surface. Additionally, a system is needed that can rapidly heat the test specimen through representative temperature cycles without using a furnace, which allows the camera to get close enough to the specimen to accurately capture in real-time (in-situ) the location and shape of the moving liquid front. Image-processing software can then be used to calculate the velocity of the moving liquid front, as well as other parameters of importance, such as fillet contact angles and shape of the fillet""s meniscus.
Against this background, the present invention was developed.